In order to answer Question 30 on the application for Obamacare, you may need a crystal ball.

The first half of the question simply asks for this year’s income. However, the second part asks what that income will be next year, "if you think it will be different."

In today’s economy, when workers cobble together income from various jobs that ebb and flow, that’s nearly impossible for some to answer.

Making matters worse, their answer could come back to bite them at tax time. That’s because people with fluctuating income may end up having to pay back subsidies they receive.

The only way to avoid this quandary is to notify the government as soon as possible if you wind up making more than you anticipated.

The Obamacare "subsidy" is actually a tax credit — money deducted straight from the federal tax you owe. Normally tax credits don’t kick in until you file your taxes.

Obamacare comes with a helpful twist, however. Realizing that people probably can’t pay for health insurance up front, you may use the credit monthly if you choose.

A raise could cost you

In fact, this is one of your first decisions if you qualify for a subsidy under the Affordable Care Act: Do you want to pay for your health insurance every month, then get your subsidy back in one lump sum in a tax refund? Or do you want the feds to send your monthly stipend directly to your insurance company?

Experts anticipate most people will choose to get the payments up front.

If your income drops because you lose your job, you can always apply for the insurance stipend.

If you get a better-paying job, however, you may find yourself in a pickle, said William McDevitt, an accountant with the East Brunswick firm of Wilkin & Guttenplan,

If that new job comes without health benefits, you won’t need to switch policies. However, you’ll no longer qualify for a big subsidy. In fact, you may not qualify for any subsidy at all.

A California study predicts more than half of applicants will have swings of income at least 20 percent above or below what they’re making now. Roughly a fifth will make less; a third will make more, according to the California Program on Access to Care.

In all, four out of five applicants will see some fine-tuning on their taxes.

If you continue to take the higher subsidy to which you’re no longer entitled, you’re going to have to pay some of it back. "You can say, ‘Well, shame on them,’ for accepting subsidies they no longer deserve, but they may not know about it, and many don’t even understand it," said McDevitt.

"The IRS looks at the actual income for the tax year," not the guesstimate on your insurance application, said Linda Schwimmer of the New Jersey Health Care Quality Institute, a nonprofit research group.

"You’ve got to be very vigilant about income changes and report them as soon as possible," said Jeff Brown, director of policy and program at the Institute. "People are almost going to have to become their own accountants." Once you disclose your new salary, the feds are supposed to adjust the subsidy in a "timely" manner. If you get a raise, it will drop. If the raise is big enough, it may stop entirely.

At tax time, you’ll be glad you took this step.

It pays to be vigilant

By contrast, if you stay mum about your new paycheck until you file your taxes, you’ll find your refund diminished, gone entirely, or actually turned into a tax bill.

Given the hardship this will cause some, the new law sets a limit, or "cap," on how much anyone will have to repay, said Justine Ceserano, New Jersey state director of EnrollAmerica.

The poorest taxpayers will have to repay a maximum of $300 (individual) or $600 (family) of the excess subsidy they received. The middle group, based on income, will have to repay up to $750 (individual) or $1,500 (family), while the group making the highest incomes will have the cap set at $1,250 (individual) or $2,500 (family.)

Schwimmer said the most problematic scenario is one in which the applicant has a low income at the time of enrollment, then finishes the year with a high income. Once an individual tops about $45,000 in annual income, the subsidy vanishes. So any previous subsidy payments must be repaid. As a kicker, there is no cap, or limit. "The lack of a cap for these families places them at a greater risk if they don’t monitor the situation and save for it," Schwimmer said.

So it’s best to be forthcoming about any raise, even if it means paying more for your health insurance, McDevitt said. He likens it to the old TV commercial for oil filters in which the mechanic says, "Pay me now, or pay me later."